Transmitter optical signal to noise ratio improvement through receiver amplification in single laser coherent systems

ABSTRACT

A transceiver having an improved transmitter optical signal to noise ratio, and methods of making and using the same.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/170,850, filed Feb. 28, 2019, now allowed, which is a continuation ofU.S. patent application Ser. No. 15/405,516, filed Jan. 13, 2017, nowU.S. Pat. No. 10,135,536, which is a continuation of U.S. patentapplication Ser. No. 14/794,889, filed Jul. 9, 2015, now U.S. Pat. No.9,571,200, which are hereby incorporated by reference herein in theirentirety.

FIELD OF THE INVENTION

The invention relates to optical communication systems in general andparticularly to a receiver for an optical communication system.

BACKGROUND OF THE INVENTION

In coherent optical communication systems, optical amplifiers are oftenused at the output of the transmitter (Tx). Additionally, in power- andcost-constrained systems, a single laser may be shared between thetransmitter and local oscillator (LO) in the receiver (Rx). In such asystem using a Tx optical amplifier, the output Tx optical signal tonoise ratio (OSNR) is typically limited by amplified spontaneousemission (ASE) noise due to the optical amplifier. The OSNR can beimproved by reducing the insertion loss of the transmitter or byincreasing the laser power dedicated to the transmitter. Typically, bothof these parameters are fixed.

FIG. 1 is a schematic block diagram 100 of a prior art coherent opticaltransceiver 102, in which in-line power monitor photodiodes and fiberoptic splices are omitted. A WDM coupler couples the light from pumplaser 114 into the Tx EDFA 106. The Tx photodiode (PD) 116 is used tocontrol the Tx EDFA 106 gain via a feedback loop 118 to control the pumplaser 114 current. The light from the tunable laser 108 is sharedbetween Tx 110 and Rx 112 and its splitting ratio is dictated by therequired LO power P_(LO) as explained below. The incoming receiversignal power P_(r) and the target photo current I_(r)∝√{square root over(P_(LO)P_(r))} determine the LO power P_(LO) and, therefore, the Txpower P_(Tx). As shown in FIG. 1, an electrical TX input 120, an opticalTx output 122, an optical RX input 130 and an electrical Rx output 132are present.

The proportion of the system laser power dedicated to the Tx, P_(Tx), isdictated by what is needed for the LO in the receiver. The required LOpower, P_(LO), is determined by the amount of gain one needs in thereceiver. In coherent intradyne detection, the input received opticalsignal, P_(r), is mixed with a higher powered local oscillator P_(LO)near the same frequency in order to boost the power in the mixingproducts at the intermediate frequencies ω_(IF)=ω_(r)−Ω_(LO), whereω_(r) is a frequency within the signal bandwidth. Given a signal inputpower P_(r), detector responsivity R, and target current outputI_(r)∝R√{square root over (P_(LO))}√{square root over (P_(r))}, the LOpower is fixed.

There is a need for systems that provide an optical communication systemwith improved optical signal to noise ratio.

SUMMARY OF THE INVENTION

According to one aspect, the invention features an optical transceiverfor receiving an electrical input signal and outputting an opticaloutput signal responsive to the electrical input signal, and forreceiving an optical input signal and outputting an electrical outputsignal responsive to the optical input signal, comprising:

a first shared optical source configured to provide pump light;

a first splitter configured to split the pump light into a firsttransmitter pump light portion and a second receiver pump light portion,the first splitter configured to split the pump light into desiredproportions based on the first transmitter pump light portion requiredfor amplification of the optical output signal;

a first optical amplifier configured to receive the first transmitterpump light portion and amplify the optical output signal;

a second optical amplifier configured to receiver the second receiverpump light portion and amplify the optical input signal;

a second shared optical source configured to generate input light;

a second variable splitter configured to split the input light intosignal light and local oscillator (LO) light, the second variablesplitter configurable to provide a splitting ratio based on the LO lightrequired for amplification of the optical input signal, and the secondreceiver pump light provided by the first splitter;

an optical transmitter configured to receive the signal light and theelectrical input signal and to transmit the optical output signal; and

a coherent optical receiver configured to receive the input opticalsignal and the local oscillator (LO) light and to output the electricaloutput signal;

wherein the splitting ratio is configurable based on the desiredproportions of the first splitter to trade off receiver dynamic rangeand transmitter optical signal to noise ratio.

In one embodiment, the first coherent optical source is a tunable laser.

In another embodiment, the second coherent optical source is a pumplaser.

In still another embodiment, at least one of the first optical amplifierand the second optical amplifier is a fiber amplifier.

In yet another embodiment, the fiber amplifier is an EDFA.

In a further embodiment, the first optical feedback loop controlcomprises a photodetector and an optical feedback loop controller.

In yet a further embodiment, the optical intensity controller is avariable optical attenuator.

In an additional embodiment, the second optical amplifier configured toreceive an input optical signal from the second coherent optical sourceis in optical connection to the second coherent optical source by way ofan optical splitter.

In one more embodiment, the improved optical signal to noise ratio is animproved transmitter optical signal to noise ratio.

In still another embodiment, the transceiver having an improved opticalsignal to noise ratio further comprises a variable power splitterconfigured to receive optical power from the first coherent opticalsource and configured to provide illumination with intensity P_(T) tothe transmitter and illumination with intensity P_(LO) to the receiverso that the ratio between the intensity P_(T) and the intensity P_(LO)can be set to a desired value.

According to another aspect, the invention features an improvedtransceiver having an improved optical signal to noise ratio, thetransceiver having a transmitter having an electrical transmitter inputport and an optical transmitter output port, a receiver having anoptical receiver input port and an electrical receiver output port, afirst coherent optical source shared between the transmitter and thereceiver and configured to provide illumination with intensity P_(T) tothe transmitter and illumination with intensity P_(LO) to the receiver,and a second coherent optical source in optical communication with theoptical transmitter output port by way of an optical coupler, theoptical transmitter output port comprising a first optical amplifier,the second coherent optical source having a first optical feedback loopcontrol; wherein the improvement comprises: a second optical amplifierconfigured to receive an input optical signal from the second coherentoptical source, and to provide amplified illumination to an opticalintensity controller, the optical intensity controller configured toprovide controlled intensity illumination to the receiver, the opticalintensity controller being part of a second feedback loop.

According to yet another aspect, the invention relates to a method ofmaking a transceiver having an improved optical signal to noise ratio.The method comprises the steps of: providing a transmitter having anelectrical transmitter input port and an optical transmitter outputport; providing a receiver having an optical receiver input port and anelectrical receiver output port; providing a first coherent opticalsource shared between the transmitter and the receiver and configured toprovide illumination with intensity P_(T) to the transmitter andillumination with intensity P_(LO) to the receiver; providing a secondcoherent optical source in optical communication with the opticaltransmitter output port by way of an optical coupler, the opticaltransmitter output port comprising a first optical amplifier, the secondcoherent optical source having a first optical feedback loop control;and providing a second optical amplifier configured to receive an inputoptical signal from the second coherent optical source, and to provideamplified illumination to an optical intensity controller, the opticalintensity controller configured to provide controlled intensityillumination to the receiver, the optical intensity controller beingpart of a second feedback loop.

According to still another aspect, the invention relates to a method ofusing a transceiver having an improved optical signal to noise ratio.The method comprises the steps of: providing a transmitter having anelectrical transmitter input port and an optical transmitter outputport; providing a receiver having an optical receiver input port and anelectrical receiver output port; providing a first coherent opticalsource shared between the transmitter and the receiver and configured toprovide illumination with intensity P_(T) to the transmitter andillumination with intensity P_(LO) to the receiver; providing a secondcoherent optical source in optical communication with the opticaltransmitter output port by way of an optical coupler, the opticaltransmitter output port comprising a first optical amplifier, the secondcoherent optical source having a first optical feedback loop control;providing a second optical amplifier configured to receive an inputoptical signal from the second coherent optical source, and to provideamplified illumination to an optical intensity controller, the opticalintensity controller configured to provide controlled intensityillumination to the receiver, the optical intensity controller beingpart of a second feedback loop; operating the first coherent opticalsource and the second coherent optical source; applying at least one ofan electrical input signal to the electrical transmitter input port andan optical input signal to the optical receiver input port; andperforming at least one of: transmitting an optical signal responsive tothe electrical input signal; and receiving an electrical signalresponsive to the optical input signal.

The foregoing and other objects, aspects, features, and advantages ofthe invention will become more apparent from the following descriptionand from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the invention can be better understood withreference to the drawings described below, and the claims. The drawingsare not necessarily to scale, emphasis instead generally being placedupon illustrating the principles of the invention. In the drawings, likenumerals are used to indicate like parts throughout the various views.

FIG. 1 is a schematic block diagram of a prior art coherent opticaltransceiver.

FIG. 2 is a schematic diagram of one embodiment of an apparatus having asingle pump laser source for two Erbium-doped fiber coils according toprinciples of the invention.

FIG. 3 is a graph showing the OSNR gain vs LO power relative to a 50:50split of a 16 dBm output power tunable laser.

FIG. 4 is a schematic diagram of one embodiment of an apparatus having asingle pump laser source for two Erbium-doped fiber coils and a tunablelaser with a variable power splitter according to principles of theinvention.

DETAILED DESCRIPTION Acronyms

A list of acronyms and their usual meanings in the present document(unless otherwise explicitly stated to denote a different thing) arepresented below.

ASE Amplified Spontaneous Emission

CMOS Complementary Metal-Oxide-Semiconductor

DBR Distributed Bragg Reflector

DSP Digital Signal Processor

ECL External Cavity Laser

EDFA Erbium Doped Fiber Amplifier

E/O Electro-optical

FDTD Finite Difference Time Domain

FFE Feed-Forward Equalization

FOM Figure of Merit

FSR Free Spectral Range

FWHM Full Width at Half Maximum

LIV Light intensity(L)-Current(I)-Voltage(V)

OSNR Optical Signal to Noise Ratio

PIC Photonic Integrated Circuits

PRBS Pseudo Random Bit Sequence

PDFA Praseodymium-Doped-Fiber-Amplifier

RSOA Reflective Semiconductor Optical Amplifier

Rx Receiver

SMSR Single-Mode Suppression Ratio

Tx Transmitter

VOA Variable Optical Attenuator

WDM Wavelength Division Multiplexing

This invention involves sharing the pump laser power used to operate theTx fiber amplifier with an additional Rx fiber amplifier in order toimprove the output Tx power and, as a result, the Tx OSNR of a coherentoptical transceiver using a single pump laser source. Since the outputsignal power and the required gain of the Rx optical amplifier is muchlower than those of the Tx optical amplifier, only a small fraction ofthe pump laser power is required for the Rx optical amplifier.

Example Implementation

FIG. 2 is a schematic diagram of one embodiment of an apparatus having asingle pump laser source for two amplification fiber coils (in theembodiment illustrated, the amplification fiber coils are Erbium-dopedfiber amplifier coils) according to principles of the invention. Themethod of using a single pump laser source for two amplification fibercoils is also described. The required gain and output power in the Rxoptical amplifier is much lower than in the Tx optical amplifier.Therefore, the optical splitter need only tap off a small fraction ofthe pump laser power for the Rx optical amplifier. In other embodiments,amplifiers other than EDFA amplifiers may be used.

As illustrated in FIG. 2 is a schematic block diagram 200 of a prior artcoherent optical transceiver 202, in which in-line power monitorphotodiodes and fiber optic splices are omitted. The pump laser 214provides illumination to an optical splitter 250 which can split theillumination in desired proportions. One portion of the illuminationfrom the pump laser 214 is provided to the WDM coupler 204 that couplesinto the optical Tx output. The Tx photodiode (PD) 216 is used tocontrol the Tx EDFA 206 gain via a feedback loop 218 that samples theoutput of Tx EDFA 206 and controls the current driving the pump laser214. Another portion of the illumination from the pump laser 214 isprovided by signal splitter 250 to the WDM coupler 252 that feeds anErbium-doped fiber 256 which provides amplification of illuminationentering on the Optical Rx input 230. The amplified light exits theErbium-doped fiber 256 and is fed into a VOA 256 that in turn feeds intothe Rx 212. The VOA 256 is controlled by a feedback loop 258 thatsamples the output of Rx 212, and provides a control signal to the VOA256.

The light from the tunable laser 208 is shared between Tx 210 and Rx 212and its splitting ratio is dictated by the required LO power P_(LO). Asshown in FIG. 2, an electrical TX input 220, an optical Tx output 222,an optical RX input 230 and an electrical Rx output 232 are present.

An increase of the signal power, P_(r), at the receiver through anadditional optical pre-amplifier with gain G_(r) in front of thereceiver allows one to lower the required LO power P_(LO) by the samefactor G_(r). In amplifier-noise limited transmission, for example inmetro and long haul links using optical amplifier chains, the decreasein signal-to-noise ratio (SNR) at the receiver associated with theadditional optical pre-amplifier noise is marginal as can be seen by theFriis formula:

$F_{total} = {F_{1} + \frac{F_{2} - 1}{G_{1}} + \frac{F_{3} - 1}{G_{1}G_{2}} + \frac{F_{4} - 1}{G_{1}G_{2}G_{3}} + \ldots\mspace{14mu} + \frac{F_{n} - 1}{G_{1}G_{2}\mspace{14mu}\ldots\mspace{14mu} G_{n - 1}}}$

Here, F_(total) is the noise figure of the complete amplifier chain withamplifiers exhibiting individual gains G_(i) and noise figures F_(i).The contribution of the last amplifier, in our case the additionaloptical pre-amplifier for the receiver, is strongly suppressed.

However despite of the benefit of an optical pre-amplifier, it is oftennot desirable to duplicate the pump laser due to cost and spaceconcerns. Additionally, the electrical power required by an additionalpump laser would be disadvantageous in small form factor devices, suchas the CFP2 form factor.

While the Tx EDFA gain is controlled via the EDFA pump power, the gaincontrol of the Rx EDFA fed by the same pump would require an additionalvariable optical attenuator (VOA) for the Rx EDFA pump light.

In the present invention, one uses a free running (not gain-controlled)Rx EDFA and one controls the Rx EDFA output power with a VOA. The Rxinput VOA can more easily be implemented in the receiver input signalpath.

For purposes of elucidation of features of the invention, one canstipulate that the coherent transceiver block outputs an averagemodulated Tx output power of −17 dBm. In conventional opticalcommunication systems the typical system output power requirements areat least +1 dBm. Therefore, the Tx optical amplifier needs to supply 18dB of gain. This may be accomplished with 18 meters of erbium-dopedfiber and 30 mW of pump power.

In the Rx, only 5 or 10 dB of gain is desired, which can be accomplishedwith 5 meters of erbium-doped fiber and less than 10 mW of pump opticalpower. Therefore, a 40 mW pump laser can be used and a 1:4 splitter cansupply both EDFA coils.

The 16 dBm laser input was previously being split 50:50 between Tx andRx LO. With an Rx amplifier, 9 dBm could be used for the LO instead of13 dBm. This would allow the Tx to use 15 dBm, for a gain in output OSNRof 2 dB.

Those of ordinary skill in the art will understand that the numericalvalues provided in the embodiment describe represent one example, andare provided for the purpose of explanation. Actual numerical values andpower requirements may vary in different embodiments.

FIG. 3 is a graph showing the OSNR gain vs LO power relative to a 50:50split of a 16 dBm output power tunable laser. Reducing P_(LO) from 13dBm to 9 dBm allows one to increase P_(Tx) from 13 dBm to 15 dBm, thusboosting Tx OSNR by 2 dB.

As has been explained, the Rx fiber amplifier can be used to increasethe Tx power/OSNR as much as possible. This is optimal for OSNR limitedsystems.

FIG. 4 is a schematic diagram of one embodiment of an apparatus having asingle pump laser source for two Erbium-doped fiber coils and a tunablelaser with a variable power splitter according to principles of theinvention. In the embodiment illustrated, the amplification fiber coilsare Erbium-doped fiber amplifier coils. In other embodiments, amplifiersother than EDFA amplifiers may be used.

As illustrated in FIG. 4 is a schematic block diagram 400 of a prior artcoherent optical transceiver 402, in which in-line power monitorphotodiodes and fiber optic splices are omitted. The pump laser 414provides illumination to an optical splitter 450 which can split theillumination in desired proportions. One portion of the illuminationfrom the pump laser 414 is provided to the WDM coupler 404 that couplesinto the optical Tx output. The Tx photodiode (PD) 416 is used tocontrol the Tx EDFA 406 gain via a feedback loop 418 that samples theoutput of Tx EDFA 406 and controls the current driving the pump laser414. Another portion of the illumination from the pump laser 414 isprovided by signal splitter 450 to the WDM coupler 452 that feeds anErbium-doped fiber 456 which provides amplification of illuminationentering on the Optical Rx input 430. The amplified light exits theErbium-doped fiber 456 and is fed into a VOA 456 that in turn feeds intothe Rx 412. The VOA 456 is controlled by a feedback loop 458 thatsamples the output of Rx 412, and provides a control signal to the VOA456.

The light from the tunable laser 408 is shared between Tx 410 and Rx 412and its splitting ratio is controlled by a variable power splitter 460so that the ratio between the Tx power P_(Tx) (or the intensity P_(T))and the LO power P_(LO) (or the intensity P_(LO)) can be varied and canbe set to a desired value. As shown in FIG. 4, an electrical TX input420, an optical Tx output 422, an optical RX input 430 and an electricalRx output 432 are present.

In combination with the single laser transceiver and a Rx fiberamplifier, a tunable laser with a variable power splitter in thetransceiver allows for variable split ratios between the Tx power andthe LO power. This variability enables a flexible trade-off between TxOSNR and Rx sensitivity and dynamic range. In combination with avariable power splitter on the transceiver, as shown in FIG. 4, theshared pump Rx fiber amplifier provides a flexible transceiver capableof trading off Rx dynamic range vs Tx OSNR and, thus, extending therange of applications.

It is believed that for future metro and short reach networks which maybe Rx input power limited, the shared pump Rx fiber amplifier could alsobe used to increase receiver dynamic range at the expense of Tx OSNR. Bydedicating more laser power to the LO of the Rx together with theamplification of the Rx input signal, the Rx sensitivity/dynamic rangecould be significantly increased. For a 50/50 split (13 dBm LO power fora 16 dBm input power laser), the full gain (5 to 10 dB) of the Rx fiberamplifier would be usable to increase the Rx sensitivity.

Design and Fabrication

Methods of designing and fabricating devices having elements similar tothose described herein are described in one or more of U.S. Pat. Nos.7,200,308, 7,339,724, 7,424,192, 7,480,434, 7,643,714, 7,760,970,7,894,696, 8,031,985, 8,067,724, 8,098,965, 8,203,115, 8,237,102,8,258,476, 8,270,778, 8,280,211, 8,311,374, 8,340,486, 8,380,016,8,390,922, 8,798,406, and 8,818,141, each of which documents is herebyincorporated by reference herein in its entirety.

Definitions

As used herein, the term “optical communication channel” is intended todenote a single optical channel, such as light that can carryinformation using a specific carrier wavelength in a wavelength divisionmultiplexed (WDM) system. In some embodiments, the the Rx input signalcan in principle carry a large number of optical communication channels.The LO is used to select which of them shall be detected at any giventime.

As used herein, the term “optical carrier” is intended to denote amedium or a structure through which any number of optical signalsincluding WDM signals can propagate, which by way of example can includegases such as air, a void such as a vacuum or extraterrestrial space,and structures such as optical fibers and optical waveguides.

THEORETICAL DISCUSSION

Although the theoretical description given herein is thought to becorrect, the operation of the devices described and claimed herein doesnot depend upon the accuracy or validity of the theoretical description.That is, later theoretical developments that may explain the observedresults on a basis different from the theory presented herein will notdetract from the inventions described herein.

Any patent, patent application, patent application publication, journalarticle, book, published paper, or other publicly available materialidentified in the specification is hereby incorporated by referenceherein in its entirety. Any material, or portion thereof, that is saidto be incorporated by reference herein, but which conflicts withexisting definitions, statements, or other disclosure materialexplicitly set forth herein is only incorporated to the extent that noconflict arises between that incorporated material and the presentdisclosure material. In the event of a conflict, the conflict is to beresolved in favor of the present disclosure as the preferred disclosure.

While the present invention has been particularly shown and describedwith reference to the preferred mode as illustrated in the drawing, itwill be understood by one skilled in the art that various changes indetail may be affected therein without departing from the spirit andscope of the invention as defined by the claims.

What is claimed is:
 1. An optical transceiver for receiving anelectrical input signal and outputting an optical output signalresponsive to the electrical input signal, and for receiving an opticalinput signal and outputting an electrical output signal responsive tothe optical input signal, comprising: a first shared optical sourceconfigured to provide pump light; a first splitter configured to splitthe pump light into a first transmitter pump light portion and a secondreceiver pump light portion, the first splitter configured to split thepump light into desired proportions based on the first transmitter pumplight portion required for amplification of the optical output signal; afirst optical amplifier configured to receive the first transmitter pumplight portion and amplify the optical output signal; a second opticalamplifier configured to receiver the second receiver pump light portionand amplify the optical input signal; a second shared optical sourceconfigured to generate input light; a second variable splitterconfigured to split the input light into signal light and localoscillator (LO) light, the second variable splitter configurable toprovide a splitting ratio based on the LO light required foramplification of the optical input signal, and the second receiver pumplight provided by the first splitter; and an optical transmitterconfigured to receive the signal light and the electrical input signaland to transmit the optical output signal; a coherent optical receiverconfigured to receive the input optical signal and the local oscillator(LO) light and to output the electrical output signal; wherein thesplitting ratio is configurable based on the desired proportions of thefirst splitter to trade off receiver dynamic range and transmitteroptical signal to noise ratio.
 2. The transceiver according to claim 1,wherein the second optical amplifier comprises a free-running opticalamplifier configured to amplify the optical input signal; wherein thefirst optical amplifier comprises a gain-controlled optical amplifierconfigured to amplify the optical output signal; and further comprisinga first control loop configured to sample the optical output signal andto increase or decrease power to the pump light, thereby increasing ordecreasing power to the first transmitter pump light portion for thefirst optical amplifier to increase or decrease the amplification of theoptical output signal, resulting in a corresponding increase or decreasein power to the second receiver pump light portion for the secondoptical amplifier to increase or decrease the amplification of theoptical input signal.
 3. The transceiver according to claim 2, whereinthe first control loop includes a photodetector for sampling the opticaloutput signal, and a feedback loop controller.
 4. The transceiveraccording to claim 2, further comprising a variable optical attenuator(VOA) configured to attenuate the optical input signal in response toamplification of the optical input signal by the second opticalamplifier resulting from an increase in power to the pump light foramplification of the optical output signal by the first opticalamplifier.
 5. The transceiver according to claim 4, further comprising asecond control loop configured to sample a portion of an output from thecoherent optical receiver, and provide a control signal to the variableoptical attenuator for controlling optical power of the optical inputsignal received by the coherent optical receiver based on amplificationby the second optical amplifier, and an optical power of the LO light.6. The transceiver according to claim 1, wherein the second sharedoptical source comprises a wavelength-tunable laser, and the firstshared optical source comprises a pump laser.
 7. The transceiveraccording to claim 1, wherein at least one of the first opticalamplifier and the second optical amplifier comprises a fiber amplifier.8. The transceiver according to claim 1, wherein the first splitter isconfigured to split the pump light, whereby the second transmitter pumplight portion is smaller than the first receiver pump light portion. 9.The transceiver according to claim 8, wherein the first splitter isconfigured to split the pump light by a 1:4 ratio, whereby the firsttransmitter pump light portion comprises four times as much pump poweras the second receiver pump light portion.
 10. The transceiver accordingto claim 1, further comprising a first optical coupler configured tocouple the first transmitter pump light portion into the first opticalamplifier; and a second optical coupler configured to couple the secondreceiver pump light portion into the second optical amplifier.
 11. Thetransceiver according to claim 4, wherein the first control loop isconfigured to vary current to the first shared optical source independence upon power of the output optical signal.
 12. The transceiveraccording to claim 11, wherein the second variable splitter isconfigured to lower power of the LO light when gain in the secondoptical amplifier increases.
 13. A method of operating an opticaltransceiver comprising an optical transmitter for transmitting an outputoptical signal, and a coherent optical receiver for receiving an inputoptical signal and generating an output electrical signal, the methodcomprising: generating input light with a first shared optical source;generating pump light with a second shared optical source; splitting thepump light generated by the second shared optical source with a firstsplitter into first and second portions, and providing the first portionto a first optical amplifier based on required amplification of theoptical output signal, and a smaller second portion to a second opticalamplifier to amplify the input optical signal, thereby improving outputoptical signal to noise ratio (OSNR) for the optical transmitter byproviding a greater portion of the pump light from the second sharedoptical source to the first optical amplifier in the opticaltransmitter; and splitting the input light from the first shared opticalsource into an optical signal light and local oscillator (LO) light witha second variable splitter, and providing the optical signal light tothe optical transmitter and the LO light to the coherent opticalreceiver, with a splitting ratio based on the LO light required foramplification of the input optical signal, and the second portionprovided by the first splitter; controlling the splitting ratio of thesecond variable splitter to provide a desired receiver dynamic range andtransmitter OSNR.
 14. The method according to claim 13, wherein thefirst optical amplifier comprises a gain-controlled optical amplifierconfigured to amplify the output optical signal; wherein the secondoptical amplifier comprises a free-running optical amplifier configuredto amplify the input optical signal; and further comprising controllingamplification of the first optical amplifier with a first control loopbased on sampling the output optical signal.
 15. The method according toclaim 14, further comprising attenuating the input optical signalreceived by the coherent optical receiver using a variable opticalattenuator in a second control loop in dependence on amplification ofthe second optical amplifier, an optical power of the LO light, and theoutput electrical signal.
 16. The method according to claim 14, whereinthe controlling amplification of the first optical amplifier comprisesvarying pump power generated by the second shared optical source independence upon a sample of the output optical signal at an output ofthe first optical amplifier.
 17. The method according to claim 13,wherein the second variable splitter is configured to lower power of theLO light when gain in the second optical amplifier increases.